1. Field of the Invention
The invention pertains to lubricants for the warm-forming of metals, particularly titanium, steel, and aluminum, and magnesium and their alloys, and paintable products obtained thereby.
2. Background Art
Many stamped products are cold formed, i.e., stamped between male/female mold halves at temperatures close to ambient. For parts with modest depth and gentle contours, the method works well. However, for parts of deeper section, particularly those of complex shape, this method has proven inadequate. In parts of aluminum and magnesium in particular, the metal tends to gall, tear, fracture, or exhibit poor surface quality. Thus, warm-forming techniques have been developed. Warm forming is well known in the art. See, e.g. the HANDBOOK OF ALUMINUM, G. E. Totten et al., CRC Press, 2003, which is incorporated for this purpose herein.
Warm-forming takes place at considerably elevated temperatures, yet at temperatures below the melting point of the alloy being formed. With aluminum alloys, for example, temperatures of the alloy precursor may range from 600° F. to 1000° F. Under these conditions, the alloy loses considerable strength and modulus, but becomes more ductile. These qualities are needed to form deep sections. However, aluminum, and in particular magnesium, are generally warm formed at considerably lower temperatures, for example in the range of 200° C. to 320° C. Magnesium warm forms well at 225° C., since at this temperature, additional slip planes are generated in the lattice.
However, the warm-forming process is also subject to galling and poor surface quality. These problems may be eliminated through the use of a suitable lubricant. However, because of the elevated temperatures, any lubricant must not only be thermally stable, but must also retain its lubricant qualities at the forming temperatures. Thus, lubricant oils, while somewhat effective in cold-forming, loose their viscosity and a great deal of their lubricity at elevated temperatures.
Many deep drawn parts, such as those for the transportation sector, must subsequently be painted. It is important that the lubricant may either easily be removed by simple, cost-effective processes, or preferably, need not be removed at all, while not impairing paintability. Polyethylene waxes and powders, for example, provided heating time is short, may serve as lubricants. However, polyethylene is not paint receptive, and must be removed. Aqueous washes are incapable of thorough removal, requiring a warm organic solvent rinse. Such rinses are environmentally disfavored, and disfavored for safety and cost reasons as well.